Nowadays, different methods for marine survey of carbohydrate deposits are widely used. These methods are typically associated with:—influence of electromagnetic field pulses upon sea bottom;—registration of changes of electromagnetic parameters of near-bottom strata; and—analysis of obtained data for detection of existing anomalies and determination of their nature (RU 2236028, 2004; SU 1122998, 1984; SU 1798666, 1996; SU 1434385, 1988; U.S. Pat. Nos. 4,298,840, 1981; 4,617,518, 1986). Such methods are carried out with the help of different apparatus research complexes.
The most universal and prospective method for sea bottom survey utilizing vessels is the method of induced polarization (IP) (RU 2236028; RU 2253881, SU 1798666; SU 1434385; U.S. Pat. Nos. 4,298,840; 4,617,518), that allows taking into account both conductivity and chargeability of a sea bottom section during plotting the section profile.
There are known methods and apparatus described in RU2236028 and RU 2253881, in which the environment is electrically excited by a horizontal generator line towed behind a vessel, while primary and secondary signals are measured by a towed receiving line with the length from 500 up to 1000 meters.
This apparatus is practically inapplicable in shallow water conditions, since the vessel, which the complex is based upon, must be placed at a sufficient distance from the shore in a zone of significant depths. Moreover, obtaining profile data using such method is limited, as it provides only two variants: the excitation of environment at a fixed distance symmetrically in reference to a sounding point (i.e. a point of generation of probe signals), or immediately in the sounding point using two measuring stations. For example, the research complex of RU 2236028 uses unipolar current pulses and the generator dipole is located along the axis of a measurement line.
There is known a research complex (RC) described in RU0048645 including a vessel carrying a generator and a block of excitation field formation (BFF) that allows generating pulses in a discrete mode, a measuring apparatus, and auxiliary devices.
According to RU0048645, the BFF is connected with a vertical dipole having generator electrodes immersed into water, wherein the lower end of the dipole is located at a distance no more than 100 meters from the sea bottom. For registration of the signals, a set of bottom stations (BS) is used, including typical electric or magnetic bottom stations with flexible rods serving for placement of receiving electrodes. The stations are located in such a way that at least three stations are positioned within an area of possible deposits, while some of the stations are positioned beyond that area.
According to RU0048645, the RC also includes: auxiliary devices, in particular, a block of self-emersion of the bottom stations; a non-irradiating dummy load device, ensuring dissipation of the generator's energy in intervals between the pulses and consisting of pairs of electric dipoles with different directions and equal moments; and equipment for determination of the vessel's location, of the sea depth, etc.
The measurement method comprises a step of synchronization of the BFF's and the bottom stations' clocks at the moment when the vessel comes to the point of profile beginning, before placement of the bottom stations. The bottom stations are placed along the measurement profile in predetermined points in such a way that no less than three bottom stations would be located in the area of possible deposits, and a part of stations would be located beyond its boundaries.
After the stations have been placed, the vessel moves to the point located at a distance no less than the sea depth from the profile beginning, the generator line is lowered vertically, in order to position the lower generator electrode at a distance no more than 100 meters from the sea bottom; then the BFF is started, which forms bipolar pulses with pauses, producing a polarizing effect on the sea bottom strata.
In the pauses, the non-irradiating dummy load device is connected to the vessel's generator to reduce a surge of electric current. The block of current measurement (BCM) executes the current measurement in the dipole with a discreteness determined by a program both during the pulses and in the pause between the pulses with fixation of time of the beginning and time of the end of each pulse. The bottom stations execute signal registration with the same discreteness that in the BFF both during the pulses and in the pause between the pulses. Based on obtained data characterizing both conductivity and chargeability of the sea bottom strata, the profile sections are built, on the base of which a conclusion on presence or absence of oil-gas deposits is made.
The disadvantage of this solution was its inapplicability for shelf transit zones because of the necessity of using the vertical generator line with a length from several tens up to several hundred meters, which is practically impossible in case of sea depths not exceeding 10 meters.
The closest to the claimed invention are a method and an apparatus complex for marine oil-gas survey described in RU2375728 earlier developed by the instant authors.
According to RU2375728, a small-sized vessel tows an electro-surveying array with a total length up to 2000 meters including a generator line and a receiving line. For the generator line having a length of 300-500 meters, bipolar current pulses separated by pauses therebetween are formed. The receiving line includes minimum three electrodes located along the line with an equal step that allows measuring a first potential difference and a second potential difference (analogues of spatial derivatives), both during a pulse of current and a pause between such pulses.
The aforesaid method is characterized in that the source of information on strata properties utilizes both: data on the strata's conductivity and the strata's polarization characteristics, in particular, calculated as continuous measurements of first potential differences and second potential differences of the electric field both during the passage of current pulses and in the pause therebetween in a wide spatial-time domain.
The aforesaid method envisages: towing the generator line behind the vessel on the sea surface; exciting the electric field by alternating-sign (bipolar) pulses of a rectangular form; registering time series of signals with the help of multi-channel bottom stations equipped with receiving lines including at least three electrodes located at a distance of 50-500 meters from one another; registering time series of the first potential differences and the second potential differences of the electric field between the electrodes both during the passage of current pulses and in the pause between the pulses; for interpretation of information on the electric field, using data obtained both during the current pulses and in the pause therebetween, in a wide spatial-time domain; and determining not only the environment resistance but also its polarization characteristics.
In general, the above described method is suitable for finding anomalies of induced polarization (IP). It ensures high work productivity. However, it has a number of essential disadvantages. For example, application of the electro-surveying array of a significant length doesn't allow executing measurements near the shore, and moreover in a land-sea transition zone. Furthermore, focusing on finding only IP anomalies results in drilling unprofitable or diluted deposits. This means that the aforementioned method does not provide a suitable precision of prognosis for presence of carbohydrate deposits in near shore areas and in a land-sea transition zone.